A Re-Examination of Failure Analysis and Root Cause Determination

15• Tensile stress.Cylindrical specimens will have a “cup and cone” configuration, as shown above on theright, while the fracture surface on thick specimens will be generally perpendicular to theprincipal stress direction, as seen in the bolt in the illustrations above.B. Brittle Fracture835xPipe ExplosionBrittle fractures are characterized by rapid crack propagation without appreciable plasticdeformation. If brittle fractures occur across particular crystallographic planes they arecalled Tran crystalline fracture. If along grain boundaries they are called intergranularfracture. Brittle fracture is promoted by:• thicker section sizes,• lower service temperatures, and• increased strain rate.A material’s tendency to fracture in a brittle mode can be determined by measuring itsnotch ductility. The most common test for this is the Charpy V-notch test. Failure undertest condition can exhibit energy and fracture transitions. Shear fracture occurs underthe notch and along the free surfaces. Cleavage fracture occurs in the centercharacterized by a bright, shiny, faceted surface. 50% cleavage is the fracturetransition point. Cleavage fracture is caused by inability of the crystal structure to crossslip.Yield strength loading is required to initiate a brittle fracture; however, only muchlower stress may be needed to propagate it. Generally speaking, body-centered cubicmetals exhibit a ductile to brittle transition over a relatively narrow temperature range.The Drop Weight Test defines the nil-ductility transition temperature and is very usefulfor determining the brittle fracture susceptibility of low-strength steels. Linear elasticfracture mechanics evaluates structural reliability in terms of applied stress, crack lengthand stress intensity at the crack tip.7960